1. Field of the Invention
The present invention relates to an anisotropic etching method and apparatus, such as a method and apparatus for forming contact holes in the process of manufacturing semiconductor devices.
2. Description of the Related Art
In the manufacturing process of a semiconductor device or an LCD, anisotropic etching is performed to form a pattern on a substrate itself or on a thin film on the substrate. For example, when contact holes are to be formed in an SiO.sub.2 film formed on an Si wafer, i.e., a substrate, processing is performed in the following rough process order. A photoresist is coated on an SiO.sub.2 film to form a resist film. The resist film is exposed using an exposure mask having a predetermined pattern. The exposed resist film is developed to leave the patterned resist film. The SiO.sub.2 film is anisotropically etched using the patterned resist film as a mask.
This anisotropic etching is generally performed as dry etching using an active gas plasma at a reduced pressure. As one of apparatuses for performing the etching as described above, a parallel-plate type plasma etching apparatus is known. This apparatus comprises two opposing electrodes arranged in a process chamber. A target substrate is generally placed on the lower electrode, and an RF power supply is connected to the upper electrode. A parallel-plate type plasma etching apparatus having a structure in which the upper electrode is used as a header for supplying a process gas is also known, as shown in Unexamined Japanese Patent Application No. 61-174721. In this structure, a large number of gas supply holes are formed in the lower surface of the upper electrode, and the process gas is showered on a target substrate.
In the plasma etching apparatus in No. 61-174721, a radius R of a hole forming region on the upper electrode is specified by an equation: R=-AG+R0, where R0 is the radius of a target substrate (wafer), G is the distance between the electrodes, and A is a constant. This equation is determined to improve planar uniformity of etch rates on a target substrate. This publication describes the following. As shown in FIG. 3, when the radius R is larger than the value specified by this equation, the etch rate of the peripheral portion of the wafer is higher than the etch rate of the central portion of the wafer; when the radius is smaller than the value, the etch rate of the central portion of the wafer is higher than the etch rate of the peripheral portion of the wafer. In short, since a rate of supplying the process gas to the periphery of the wafer is higher as the radius R is larger, the etch rate of the peripheral portion of the wafer is higher, and the etch rate is lower as the radius R is smaller.
As shown in U.S. Pat. No. 4,908,095 (issued on 03/13/1990), a structure for cooling a shower type upper electrode is proposed. Although the details of the structure are to be described in the detailed description of the preferred embodiment, cooling of the upper electrode enhances an effect of decreasing a deposition amount of reaction products on a target substrate. However, in extensive studies on apparatuses of this type, several items to be considered have been found out.
The first item is planar uniformity of a degree of etching anisotropy on a target substrate. That is, degrees of etching anisotropy of the central and peripheral portions of the target substrate tend to be different from each other. Therefore, for example, when contact holes are formed at the central and peripheral portions of the wafer, the inclination angles or taper angles of the side walls of the contact holes are different from each other.
The second item is the inclination angle of a side wall, e.g., a contact hole wall surface, of an object to be etched formed by the anisotropic etching. This angle which is close to almost 90.degree. is an important factor for obtaining high-quality products.